It is well known that organotin compositions, including organotin oxides, hydroxides, alkoxides and carboxylates are effective as catalysts in the manufacture of polyester resins and polyester-containing compositions. The use of tin catalysts in the esterification of polyesters is disclosed by Caldwell in U.S. Pat. No. 2,720,507, by Dombrow et al. in U.S. Pat. No. 3,162,616, by Allison et al. in U.S. Pat. No. 3,345,339, by Cook in U.S. Pat. No. 3,716,523 and by Jackson, Jr. et al. in U.S. Pat. No. 4,554,334. The organotin catalysts decrease the time required to complete the reaction. The use of non-toxic organotin esterification catalysts in the production of polyesters to produce non-toxic compositions is discussed by Larkin et al. in U.S. Pat. No. 4,970,288.
Polyesters are formed by the condensation of a dibasic or polybasic acid and a dihydric or polyhydric alcohol to form a series of ester linkages. Optionally, a tri-functional or polyfunctional alcohol or acid functional branching agent may be used to enhance the properties of the polyester or polyester-containing material formed from the polyester. Also, optionally, monofunctional monomers such as benzoic acid or stearic acid can be used to control molecular weight.
Theoretically, one equivalent of polyol is utilized for each equivalent of acid. However, a hydroxyl-terminated polyester is prepared using an excess of polyol and a carboxyl-terminated polyester is prepared using an excess of acid. When a carboxyl-terminated polyester is prepared in a two-stage or multi-stage reaction, it is common to use excess polyol in the first stage and react this hydroxyl-terminated polyester with an excess amount of acid or anhydride in the second stage. The advantage of a two-stage synthesis, as disclosed in the above patents, is that the polyester can be prepared having excellent color and/or can be produced in less time than by using a one-stage reaction.
Esters can also be prepared by transesterification reactions. When using the ester-interchange method the long chain in the polyester is built up by a series of ester interchange reactions wherein the glycol displaces an ester to form the glycolester. Included are the reactions between two esters to yield two new esters and also include the transesterification reactions where the components of the esters involved are polyhydroxy alcohols and polybasic acids.
As discussed in the above patents, the polyester is prepared by heating the condensing mixture at temperatures of at least about 170.degree. C. up to about 275.degree. C. or higher in order to maintain the fluid state. The reaction can be performed above atmospheric pressure, up to about 20 psig (14062 kg/m.sup.2) or higher, at atmospheric pressure or under vacuum to about 15 mm Hg or lower. The esterification reaction can be conducted in the presence of a suitable solvent such as toluene or xylene and the like. Nitrogen, argon, helium or any other suitable gas may be introduced into the reactor to keep air out of the reactor or to facilitate the removal of water, low-boiling alcohol, mixtures thereof or the like. The polyesters can also be thinned in a suitable reactive monomer such as styrene or divinyl benzene as disclosed in the above references, or mixtures thereof and the like.
It would be desirable to have available a process for the preparation of polyesters which would result in less reaction time and which would result in a polyester with improved properties such as, lower color values, reduced by-products and the like.